The present invention relates to controlled self-ignition four-stroke internal-combustion engines.
Controlled self-ignition is a well-known phenomenon in two-stroke engines. This type of combustion has advantages concerning emissions: low hydrocarbon and nitrogen oxides emissions are notably obtained. Furthermore, a remarkable cycle regularity is obtained during self-ignition combustion.
Self-ignition is a phenomenon which allows to initiate the combustion by means of the residual burnt gas remaining in the combustion chamber after combustion.
Self-ignition is achieved by controlling the amount of residual gas and mixing thereof with the fresh gas (not burnt yet). The residual gas (hot burnt gas) initiates combustion of the fresh gas by means of a combination of temperature and of the presence of active species.
In two-stroke engines, the presence of residual gas is  less than  less than inherent greater than  greater than  in the combustion. In fact, when the load of the engine decreases, the amount of fresh gas decreases, which leads to an increase in the amount of residual gas (burnt gas from the previous cycle or cycles that has not left the cylinder). Two-stroke engines thus run with an internal circulation (or internal EGR) of the burnt gas at partial load. However, the presence of this internal EGR is not sufficient to obtain the desired self-ignition running. Researchers"" work also shows that mixing between this internal EGR and the fresh gas has to be controlled and limited.
The controlled self-ignition technology applied to four-stroke engines is particularly interesting because it allows to operate this type of engine with an extremely diluted mixture, with very low fuel-air ratios and consequently ultra-low nitrogen oxides emissions.
This technology however comes up against a serious technological problem which is that, in order to obtain it without the internal EGR effect of two-stroke engines, it is necessary to either greatly increase the compression ratio of the engine (with knocking problems at high loads), or to greatly warm up the fresh gas admitted (several hundred Celsius degrees), or to combine the two phenomena.
There are solutions allowing to reduce the pressure and temperature level requirements for four-stroke engines, notably by using suitable additives added to the fuel. French patent application FR-2,738,594 illustrates a solution of this type.
For four-stroke engines, it is well-known, notably from international patent application WO-93/16,276, to combine a variable valve timing with a non-return system at the intake in order to decrease the pumping losses at partial load. This solution allows to work with the intake throttle as widely open as possible.
Patent application FR-97/02,822 filed by the claimant describes a self-ignition control in a four-stroke engine. More precisely, this document recommends, at partial load, to minimize mixing of the fresh gas with the burnt gas confined in the combustion chamber by acting on the closing of the exhaust. This solution is thus close to the  less than  less than internal greater than  greater than  recirculation technique allowing the gases to be stratified in the combustion chamber.
Patent application FR-97/11,279 filed by the claimant also aims to minimize, at partial load, mixing of the fresh gas with the burnt gas contained in the combustion chamber, in order to control and favour self-ignition combustion. However, this document proposes transferring the burnt gas from the exhaust of a cylinder to the intake of the same cylinder. This solution creates a very high dilution of the recycled burnt gas, by air, before it enters the combustion chamber, which might pose problems.
The present invention aims to achieve a very simple and therefore reliable controlled self-ignition in multi-cylinder four-stroke engines, easy to implement and favouring maximum stratification of the burnt gas in the combustion chamber. Furthermore, the temperature of the burnt gas is retained or even increased according to the invention, which favours self-combustion.
The object of the present invention is thus a controlled self-ignition combustion process for a four-stroke engine comprising several cylinders having each at least one intake port and at least one exhaust port, the ports and the closing control means being conventional, i.e. as known to the man skilled in the art. The invention can be applied to direct (DIE) or indirect injection engines.
According to the invention, the process consists, during partial load running, in transferring, via a suitable transfer means, exhaust gas from a cylinder, generally during the exhaust stroke, to another cylinder, generally during the intake stroke. The exhaust gas is sent, via a specific valve placed after the exhaust means, to the transfer means. By means of a second valve, the exhaust gas thus transferred reaches the intake line upstream from the intake means. In order to obtain a higher efficiency, it is desirable to have an intake means dedicated to the inflow of the exhaust gas in the cylinder (at least two intake means are then required) in order to decrease mixing between the fresh gas and the burnt gas.
The exhaust gas can thus be recovered from a cylinder at the end of the expansion stroke. It can also be fed into another cylinder at the beginning of the compression stroke.
The process according to the invention also consists in controlling the distribution of the exhaust gas flow between the exhaust system and the transfer means. Furthermore, the process can consist in thermally insulating and/or in warming up the exhaust gas transferred in said suitable transfer means in order to improve self-ignition.
In order to warm up the burnt gas flowing through the transfer means, catalysis means can be arranged in the transfer means. The position of the catalyst is a compromise between a position close to the intake valve of the cylinder, in order to have a higher temperature for the burnt gas as it enters the cylinder, or close to the exhaust valve, in order to facilitate initiation of the catalyst in case of cold start-ups. Besides the fact that the first function of this catalyst is to warm up the burnt gas in order to facilitate controlled self-ignition, in case of cold start-ups, it takes part, as soon as it is initiated, in the reduction of emissions during a stage where the main catalyst of the converter is generally not totally initiated. The catalysis means can comprise a catalyst mass or walls coated with catalyst. In this place, at full load, the catalyst does not receive the burnt gas, it is therefore not likely to undergo premature deterioration and creates no additional pressure drops.
In fact, at full load, the transfer means no longer communicate with the lines associated with the exhaust and intake ports of the cylinders. At full load, the configuration of the engine becomes conventional.
In order to increase the temperature of the exhaust gas at partial load, it is possible to increase the mixture strength of the exhaust gas, notably in the case of a direct injection engine. In this case, fuel injection at the end of the exhaust stroke allows the temperature of the exhaust gas to be increased by means of the reaction in the catalyst. It is conceivable to place a specific fuel injector upstream from the catalysis means.
According to an embodiment of the invention, a common line is used for transfer of the exhaust gas.
According to another embodiment of the invention, transfer of the exhaust gas is performed by means of a series of lines connecting the specific exhaust lines to the specific intake lines two by two.
The present invention also relates to a controlled self-ignition four-stroke internal-combustion engine comprising several cylinders having each at least one intake port and at least one exhaust port.
According to the invention, each cylinder also comprises a specific means allowing passage of the exhaust gas from the exhaust of a cylinder, generally in the exhaust stroke, to at least one other cylinder, generally in the intake stroke, as well as an associated transfer means, transfer taking place during partial load running.
A means intended for thermal insulation and/or heating of the transfer means can also be provided without departing from the scope of the invention.
The engine also advantageously comprises a means intended for distribution of the exhaust gas between the exhaust system and the transfer means, at partial load.
In addition to the specific valve for diverting (totally or partially) the exhaust gas towards the transfer means, the exhaust gas distribution means can comprise a throttling means arranged close to the exhaust means.
According to an embodiment, said transfer means comprises a common line.
According to another embodiment of the invention, the transfer means comprises a series of lines connecting the specific exhaust lines and valves to the specific intake lines and valves two by two.